1. Field of the Invention
This invention relates to micromirrors, and more particularly, micromirrors used in micro electrical mechanical systems (MEMS) devices.
2. Description of the Related Art
Micromirrors are used in a variety of consumer and industrial devices, including wavefront correction arrays, digital projection displays and fiber optic switching. For example, micromirrors in digital light processing (DLP) televisions are used to turn light to the projection screen on and off at the pixel level to form a projected image. In fiber optic switches, micromirrors are used to steer light from one fiber to another for reconfigurable signal routing. In wavefront-correction arrays, micromirrors are translated relative to one another to correct for wavefront distortion in a propagating optical wave.
In general, it is desirable to have a micromirror reflect light with high efficiency and high fidelity. This imposes two common and desirable design characteristics on the micromirrors used in such applications: high reflectivity at the operating wavelength and high optical figure, otherwise known as mirror flatness. To achieve high reflectivity, reflective metal films are often deposited onto the microfabricated MEMS mirror. Unfortunately, intrinsic stress associated with the thin film deposition and thermal stresses arising from differences in coefficients of thermal expansion may compromise the mirror flatness for such micromirror assemblies. For example, some micromirrors incorporate deposited metal layers on a mechanical support microfabricated from materials such as polysilicon or single crystal silicon. Intrinsic stresses created during deposition and subsequent coalescence of the metal layers may result in deformation of the mirror structure. Thermal stresses introduced by differential expansion of the reflective and support layers, respectively, when introduced to environmental heating and cooling, may similarly result in mirror deformation. The problem is exacerbated as thinner structural supports are used for the mirror surface to accomplish quicker micromirror response.
A number of solutions exist for addressing the intrinsic and thermal mismatch stresses in micromirror assemblies that may lead to loss of mirror flatness. To minimize thermally induced distortion, constraints on the operating temperature of the device may be imposed. This adds considerable system-level complexity and associated cost. Similarly, the deformation induced by the thin film layer stresses may be reduced by measures such as reducing the thickness of the reflective metal film, reducing the lateral size of the micromirror itself to reduce the bending moment caused by the stress, or by tailoring the stresses in the metal layers used for the micromirror surface to achieve a stress-neutral state. In another solution, a double-layered metallization is used to deposit the same metallization in exactly the same thickness onto both the top and bottom surface of the mirror support, so that the metallization-induced stresses are balanced. (See U.S. Pat. No. 6,618,184). In yet another solution, a stress-balancing layer is formed on a side of the mirror support opposite to that of the light reflective optical layer, with the stress-balancing layer being the same material or a different material as the light reflective optical layer. (See U.S. Pat. No. 6,639,724)
Unfortunately, for some micromirror applications, such as high-intensity projectors or those subject to illumination by moderate-to high-energy lasers, the thin metal reflective layers may not have sufficient optical durability. The ability to use thicker metal reflective layers would improve the robustness and reliability of the micromirrors relative to those using thin metal layers. The thicker metal layers would, however, impose greater stress-induced deformation to the mirror relative to the thin layers. Similarly, micromirrors used in these high-intensity applications would benefit from the lower energy absorption (higher reflectivity) provided by non-metallic, multilayer thin-film dielectric mirrors. These multilayer dielectric reflectors may be quite thick, however, and may similarly exacerbate the stress-induced deformation of the micromirror. In those applications, reducing the thickness of the micromirror surface to reduce stress-induced deflection of the entire assembly is not possible without degrading the mirror's performance in the wavelength band of interest. Also, further reduction in reflecting area of the micromirror to reduce warping introduces manufacturing challenges for the typically thick, multi-layer dielectric mirrors.
A need exists, therefore, for a structure and method to reduce the deformation of micromirrors incorporating thick or complex optical coatings such as dielectric reflectors induced by intrinsic and thermal stresses without requiring a reduction in reflecting area of such micromirrors.